1. Field of the Invention
The present invention relates generally to metabolic disease and neuropsychiatric disease and, more particularly, to stimulation of gastric and sympathetic neural tissue for the treatment of obesity and depression.
2. Related Art
Physiologic studies have demonstrated the presence of a sympathetic nervous system afferent pathway transmitting gastric distention information to the hypothalamus. [Barone, Zarco de Coronado et al. (1995). Gastric distension modulates hypothalamic neurons via a sympathetic afferent path through the mesencephalic periaqueductal gray. Brain Research Bulletin. 38: 239-51.] However, prior techniques have generally not addressed the problems associated with satiety, morbidity, mortality of intracranial modulation and the risk of ulcers. Unlike prior techniques, by specifically targeting sympathetic afferent fibers, the present invention effects the sensation of satiety and avoids the substantial risks of morbidity and mortality of intracranial modulation, particularly dangerous in the vicinity of the hypothalamus. Furthermore, this invention avoids the risk of ulcers inherent in vagus nerve stimulation.
A. Satiety. Stimulation of intracranial structures has been proposed and described for the treatment of obesity (U.S. Pat. No. 5,782,798). Stimulation of the left ventromedial hypothalamic (VMH) nucleus resulted in delayed eating by dogs who had been food deprived. Following 24 hours of food deprivation, dogs with VMH stimulation waited between 1 and 18 hours after food presentation before consuming a meal. Sham control dogs ate immediately upon food presentation. Dogs that received 1 hour of stimulation every 12 hours for 3 consecutive days maintained an average daily food intake of 35% of normal baseline levels. [Brown, Fessler et al. (1984). Changes in food intake with electrical stimulation of the ventromedial hypothalamus in dogs. Journal of Neurosurgery. 60: 1253-7.] B. Candidate Peripheral Nerve Pathways for Modulating Satiety. B1. Sympathetic Afferents The effect of gastric distension on activity in the lateral hypothalamus-lateral preoptic area-medial forebrain bundle (LPA-LH-MFB) was studied to determine the pathways for this gastric afferent input to the hypothalamus. [Barone, Zarco de Coronado et al. (1995). Gastric distension modulates hypothalamic neurons via a sympathetic afferent path through the mesencephalic periaqueductal gray. Brain Research Bulletin. 38: 239-51.] The periaqueductal gray matter (PAG) was found to be a relay station for this information. This modulation of the hypothalamus was attenuated but not permanently eliminated by bilateral transection of the vagus nerve. This modulation was, however, significantly reduced or eliminated by bilateral transection of the cervical sympathetic chain or spinal transection at the first cervical level. These signals containing gastric distension and temperature stimulation are mediated to a large degree by sympathetic afferents, and the PAG is a relay station for this gastric afferent input to the hypothalamus. For example, in the LPA-LH-MFB study, 26.1% of the 245 neurons studied were affected by gastric stimulation, with 17.6% increasing in firing frequency and 8.6% decreasing during gastric distension. [Barone, Zarco de Coronado et al. (1995). Gastric distension modulates hypothalamic neurons via a sympathetic afferent path through the mesencephalic periaqueductal gray. Brain Research Bulletin. 38: 239-51.] The response of 8 of 8 neurons sensitive to gastric distension were maintained, though attenuated after bilateral vagus nerves were cut. In 2 of these 8 cells, the effect was transiently eliminated for 2-4 minutes after left vagus transection, and then activity recovered. In 3 LH-MFB cells, two increased and the other decreased firing rate with gastric distension. Following bilateral sympathetic ganglion transection, the response of two were eliminated, and the third (which increased firing with distension) had a significantly attenuated response. [Barone, Zarco de Coronado et al. (1995). Gastric distension modulates hypothalamic neurons via a sympathetic afferent path through the mesencephalic periaqueductal gray. Brain Research Bulletin. 38: 239-51.] Vagus stimulation resulted in opposite or similar responses as gastric distension on the mesencephalic cells. B2. Vagus Nerve Afferents. Gastric vagal input to neurons throughout the hypothalamus has been characterized. [Yuan and Barber (1992). Hypothalamic unitary responses to gastric vagal input from the proximal stomach. American Journal of Physiology. 262: G74-80.] Nonselective epineural vagus nerve stimulation (VNS) has been described for the treatment of Obesity (U.S. Pat. No. 5,188,104). This suffers from several significant limitations that are overcome by the present invention.
The vagus nerve is well known to mediate gastric hydrochloric acid secretion. Dissection of the vagus nerve off the stomach is often performed as part of major gastric surgery for ulcers. Stimulation of the vagus nerve may pose risks for ulcers in patients, of particular concern, as obese patients often have gastroesophageal reflux disease (GERD); further augmentation of gastric acid secretion would only exacerbate this condition.
C. Assessment of Sympathetic and Vagus Stimulation. The present invention teaches a significantly more advanced neuroelectric interface technology to stimulate the vagus nerve and avoid the efferent vagus side effects, including speech and cardiac side effects common in with existing VNS technology as well as the potential ulcerogenic side effects. However, since sympathetic afferent activity appears more responsive to gastric distension, this may represent a stronger channel for modulating satiety. Furthermore, by pacing stimulating modulators on the greater curvature of the stomach, one may stimulate the majority of the circular layer of gastric musculature, thereby diffusely increasing gastric tone.
D. Neuromuscular Stimulation. The muscular layer of the stomach is comprised of 3 layers: (1) an outer longitudinal layer, (2) a circular layer in between, and (3) a deeper oblique layer. [Gray (1974). Gray's Anatomy. T. Pick and R. Howden. Philadelphia, Running Press.] The circular fibers, which lie deep to the superficial longitudinal fibers, would appear to be the layer of choice for creating uniform and consistent gastric contraction with elevated wall tension and luminal pressure. Therefore, modulators should have the ability to deliver stimulation through the longitudinal layer. If the modulator is in the form of an electrode, then the electrodes should have the ability to deliver current through the longitudinal layer.
Gray's Anatomy describes innervation as including the right and left pneumogastric nerves (not the vagus nerves), being distributed on the back and front of the stomach, respectively. A great number of branches from the sympathetic nervous system also supply the stomach. [Gray (1974). Gray's Anatomy. T. Pick and R. Howden. Philadelphia, Running Press.] Metabolic Modulation (Efferent) Electrical stimulation of the VMH enhances lipogenesis in the brown adipose tissue (BAT), preferentially over the white adipose tissue (WAT) and liver, probably through a mechanism involving activation of the sympathetic innervation of the BAT. [Takahashi and Shimazu (1982). Hypothalamic regulation of lipid metabolism in the rat: effect of hypothalamic stimulation on lipogenesis. Journal of the Autonomic Nervous System. 6: 225-35.] The VMH is a hypothalamic component of the sympathetic nervous system. [Ban (1975). Fiber connections in the hypothalamus and some autonomic functions. Pharmacology, Biochemistry & Behavior. 3: 3-13.] A thermogenic response in BAT was observed with direct sympathetic nerve stimulation. [Flaim, Horwitz et al. (1977). Coupling of signals to brown fat: a- and b-adrenergic responses in intact rats. Amer. J. Physiol. 232: R101-R109.] The BAT had abundant sympathetic innervation with adrenergic fibers that form nest-like networks around every fat cell, [Derry, Schonabum et al. (1969). Two sympathetic nerve supplies to brown adipose tissue of the rat. Canad. J. Physiol. Pharmacol. 47: 57-63.] whereas WAT has no adrenergic fibers in direct contact with fat cells except those related to the blood vessels. [Daniel and Derry (1969). Criteria for differentiation of brown and white fat in the rat. Canad. J. Physiol. Pharmacol. 47: 941-945.]